Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By isomerization
Reexamination Certificate
2000-06-13
2001-05-15
Knode, Marian C. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By isomerization
C585S480000
Reexamination Certificate
active
06232517
ABSTRACT:
A highly selective process is described for preparing 2,6-dimethylnaphthalene which comprises reacting a naphthalene hydrocarbon selected from naphthalene, methylnaphthalenes, dimethylnaphthalenes, trimethylnaphthalenes, polymethylnaphthalenes or their mixtures with one or more benzene hydrocarbons selected from benzene, toluene, xylenes, trimethylbenzenes, tetramethylbenzenes, pentamethylbenzene and/or hexamethylbenzene, under at least partially liquid phase conditions, in the presence of a zeolite belonging to the structural type MTW and optionally in the presence of a methylating agent.
2,6-dimethylnaphthalene is an intermediate in the synthesis of 2,6-naphthalenedicarboxylic acid, used as monomer in the preparation of PEN (polyethylnaphthalate). It is known that it can be recovered from fractions coming from the reforming of kerosine (U.S. Pat. No. 4,963,248) or from fractions of FCC oil (European Chemical News, page 30, Sep. 28, 1992). In the first case, the dimethylnaphthalenes must be separated by distillation, then, the 2,6 isomer is isolated by means of selective absorptions and/or crystallization. In the second case, there is an additional problem due to the presence of nitrogen and sulfur which poison the catalysts used for the separation and/or isomerization phases. There is also a process (U.S. Pat. No. 4,990,717; U.S. Pat. No. 5,118,892; U.S. Pat. Nos. 5,073,670; 5,030,781; 5,012,024) which, by means of alkenylation, cyclization, dehydrogenation, isomerization, leads to the selective synthesis of 2,6-dimethylnaphthalene. This high number of passages obviously represents a burden from an economical point of view and in addition, each passage or chemical reaction involves secondary reactions requiring separations to guarantee the purity of the intermediates or end-product. U.S. Pat. No. 5,043,501 describes a synthesis method of 2,6-dimethylnaphthalene in two steps. The first comprises the alkylation of an alkylaromatic with a C
5
olefin in the presence of a zeolitic catalyst, the second comprises a dehydrocyclization at 400-500° C. with a catalyst consisting of Pt/Ba/K on zeolite L, obtaining a product containing dimethylnaphthalenes which are then isomerized mainly to 2,6 isomer.
S. B. Pu and T. Inui describe in Applied Catalysis A, General 146 (1996) 305-316, the alkylation with methanol of methylnaphthalene, catalyzed by zeolites of the BEA, FAU and MTW groups, carried out without a solvent and in an exclusively gaseous phase. The best results are obtained with beta zeolite and faujasite. A. S. Loktev and P. S. Chekriy, in Zeolites and Related Micro-porous Materials: State of art 1994, SSSC vol. 84, J. Weitkamp et al. (Eds) describe the alkylation of naphthalene or methylnaphthalene with methanol catalyzed by ZSM-12, carried out in the presence of paraffinic solvents and in gaseous phase. The yields to dimethylnaphthalenes, and to 2,6 isomer in particular, are zero or in any case negligible. In addition, high quantities of heavy by-products are obtained, due to the reaction conditions and/or decomposition of the paraffinic solvents used.
U.S. Pat. No. 4,795,847 discloses a process for the preparation of 2,6-dialkylnaphthalenes which comprises the alkylation in gaseous phase of naphthalene or 2-alkyl-naphthalene with an alkylating agent in the presence of a zeolite selected from mordenite, EU-1, offretite, ZSM-12, ZSM-5 and ZSM-22. In the case of the methylation of naphthalene or 2-methyl-naphthalene the use of ZSM-5 is particularly preferred. To reduce undesired isomerization reactions which result in the formation of 1-alkyl-naphthalene, the zeolitic catalyst is previously subjected to precarbonization treatment. An example is provided of the alkylation of 2-methylnaphthalene with methanol, in gas phase, catalyzed by ZSM-5: the conversion obtained between 0.5 and 8 hours is from 5 to 7%, the yield to dimethylnaphthalenes ranges from 4 to 5% and the 2,6-dimethylnaphthalene isomer forms 50% of the dimethylnaphthalene fraction.
We have now unexpectedly found a process for preparing 2,6-dimethylnaphthalene starting from naphthalene, methylnaphthalenes, dimethylnaphthalenes, trimethylnaphthalenes and/or polymethylnaphthalenes carried out under at least partially liquid phase conditions and in the presence of suitable aromatic hydrocarbons, catalyzed by a zeolite of the structural type MTW (abbreviation IZA), which allows better results to be obtained in terms of yield, selectivity, conversion to useful products in the time unit and life of the catalyst, with respect to what is described in the prior art. In particular MTW zeolites, when used under the conditions of our invention, are more active not only than the same zeolites used under the conditions described previously, but also with respect to the BEA and MFI zeolites already described in the prior art as the best catalysts for the preparation of 2, 6 -dimethylnaphthalene.
The present invention therefore relates to a process for preparing 2,6-dimethylnaphthalene which comprises reacting a naphthalene hydrocarbon selected from naphthalene, methylnaphthalenes, dimethylnaphthalenes, trimethylnaphthalenes, pentamethylnaphthalenes, hexamethylnaphthalenes or their mixtures with one or more benzene hydrocarbons selected from benzene, toluene, xylenes, trimethylbenzenes, tetramethylbenzenes, pentamethylbenzene and/or hexamethylbenzene, under at least partially liquid phase conditions, in the presence of a zeolite belonging to the structural type MTW and optionally in the presence of a methylating agent.
The naphthalene hydrocarbon is preferably selected from naphthalene, methylnaphthalenes, dimethylnaphthalenes, trimethylnaphthalenes, or their mixtures. A particularly preferred aspect is that the reagent is naphthalene and/or methylnaphthalene, optionally mixed with dimethylnaphthalenes and/or trimethylnaphthalenes.
Fractions containing naphthalene, methylnaphthalenes and dimethylnaphthalenes, obtained by distillation from FOC (Fuel Oil Cracking) or LCO (Light Cycle Oil) cracking oils or obtained by the distillation of distilled oil from pit-coal tar, can be used directly in the process of the present invention.
According to a particularly preferred aspect of the present invention, the process for the preparation of 2,6-dimethylnaphthalene is carried out in the presence of the methylating agent which can be selected from methanol, dimethylether, dimethylcarbonate, dimethylsulfate, methyl iodide. Methanol is preferably used.
Operating according to our invention, a high selectivity is unexpectedly obtained, which, with respect to thermodynamic equilibrium values, is unbalanced towards the 2,6 isomer: there are consequently kinetic control conditions of the reaction. The molar ratio between the 2,6 and 2,7 isomers of dimethynaphthalene can also be considered as selectivity index reached with our invention. Whereas the thermodynamic ratio is about 1 (S. B. Pu and T. Inui, in Applied Catalysis A, General 146 (1996) 305-316) in the case of zeolitic catalysts with an MFI and BEA structure it is 1 and 1.3 respectively, in the case of MTW zeolites used in gas phase it is 1.2 and 1.5, in the case of MTW zeolites used according to our invention, this ratio varies from 1.9 to 2.7.
For evaluating the selectivity which can be reached with our invention, the selectivity to 2,6 or 2,6+1,6 isomer can also be considered. According to the thermodynamic equilibrium (calculated by S. B. Pu and T. Inui, in Applied Catalysis A, General 146 (1996) 305-316) there is 12% of the 2,6 isomer and 14% of the 1,6 isomer. With the process of our invention, values are registered equal to 26-35% for the 2,6 isomer and 25-32% for the 1,6 isomer, depending on the reaction conditions: in any case the values are higher than the thermodynamic ones.
Operating under the conditions of our invention, and in the presence of one or more of the particular benzene hydrocarbons selected, zeolites of the structural type MTW give better results than those of zeolites of the MFI group, not only because they are more active, with a longer life and therefore greate
Frigerio Francesco
Mansani Riccardo
Millini Roberto
Pazzucconi Giannino
Perego Carlo
Dang Thuan D.
Enichem S.p.A.
Knode Marian C.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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